So-called "floppy" disk memory systems for "desk top" sized computers are well known in the art. Such systems employ magnetic storage disks having a diameter of either 5.25 inches or 3.50 inches. Conventional magnetic storage disks for floppy disk drives have a track density ranging from forty-eight (48) to one hundred thirty-five (135) tracks per inch (TPI). In contrast, optical storage disks for optical memory systems achieve track densities greater than 15,000 TPI. The greater track density of optical disks is achieved by the use of optical servos that maintain fine positioning of the optical read/write head over the data tracks on the disk. Typically, concentric optical servo tracks are pre-recorded on the optical disk to guide the servo mechanism.
New advances in barium-ferrite magnetic media have allowed bit densities of magnetic storage disks to exceed the bit densities of optical disks. However, as mentioned above, track densities of magnetic media (48-135 TPI) are many times less than their optical counterparts. This limits the overall capacity of magnetic disks as compared to optical disks. Conventional magnetic disk systems employ a magnetic servo mechanism and magnetically pre-recorded servo tracks on the disks to guide the read/write head. Magnetic servo systems cannot provide the fine positioning that optical servo systems can provide.
Recently, floppy disk systems have been developed that combine magnetic disk recording techniques with the high track capacity optical servos found in optical disk systems. Such a system is described in AN INTRODUCTION TO THE INSITE 325 FLOPTICAL(R) DISK DRIVE, Godwin, in a paper presented at the SPIE Optical Data Storage Topical Meeting (1989). Essentially, an optical servo pattern is pre-recorded on a magnetic floppy disk. The optical servo pattern typically consists of a large number of equally spaced concentric tracks about the rotational axis of the disk. Data is stored in the magnetic "tracks" between the optical servo tracks using conventional magnetic recording techniques. An optical servo mechanism is provided to guide the magnetic read/write head accurately over the data between the optical servo tracks. By utilizing optical servo techniques, much higher track densities are achievable on the relatively inexpensive removable magnetic medium.
As mentioned, the optical servo pattern typically consists of a large number of equally spaced concentric tracks about the rotational axis of the disk. As disclosed in U.S. Pat. No. 4,961,123, each track may be a single continuous groove (FIG. 3), a plurality of equally spaced circular pits (FIG. 8), or a plurality of short equally spaced grooves or stitches (FIG. 9). Various methods and systems exist for inscribing the optical servo tracks on the magnetic medium.
For example, U.S. Pat. No. 5,067,039, entitled "High Track Density Magnetic Media with Pitted Optical Servo Tracks and Method for Stamping the Tracks on the Media," discloses a method for "stamping" the servo tracks on the magnetic medium. Essentially a master stamping disk is produced bearing a template of the optical servo pattern. This master disk is then pressed against the magnetic floppy disk under a pressure of several tons per square inch. The significant amount of pressure transfers the servo track pattern from the master disk to the floppy.
U.S. Pat. No. 4,633,451, entitled "Optical Servo for Magnetic Disks," discloses a method of providing optical servo information on a magnetic medium consisting of a multi-layer film. The optical servo tracks are formed on the multi-layer film by laser heating the structure to cause a reaction or interdiffusion to occur between layers. The reaction produces a reflectivity contrast of about eight percent (8%) between exposed and unexposed areas. Other methods for preparing the servo tracks are mentioned including contact printing, embossing, and lithography.
U.S. Pat. No. 4,961,123, entitled "Magnetic Information Media Storage with Optical Servo Tracks," discloses a preferable method and apparatus for etching the pattern on a disk using a focused beam of light. The magnetic disk is placed on a platen/spindle assembly and rotated. A beam of light is focused to a small spot on the spinning disk. The focussed beam has sufficient intensity to ablate the disk surface at the point of incidence, thereby reducing the reflectivity of the surface at that point. The beam can be left on during an entire revolution to produce a continuous groove or can be modulated on and off through one revolution to produce a stitched pattern. This method has several advantages. First, the intensity of the focussed beam of light can be adjusted for different types of magnetic media. Secondly, different stitched patterns can be etched simply by varying the on-off time of the beam or by varying the speed of rotation of the disk. Additionally, there is no need to produce a master disk, as with the stamping method.
As mentioned above, the optical servo pattern often comprises a number of equally spaced concentric optical servo tracks about the rotational axis of the disk. A single disk may have as many as 900 concentric servo tracks. Additionally, each optical servo track may be a continuous groove, or alternatively, may comprise a plurality of equally spaced stitches. When a stitched pattern is employed, the number of stitches in each optical servo track may exceed 1600 with each track having the same number of stitches. It is crucial for proper servo positioning that every stitch be sufficiently detectable by the servo optics. As mentioned, a preferred method of producing a stitched pattern is by focusing a beam of light on a rotating disk and modulating the beam on and off. The beam, when incident upon the surface of the disk and properly focused, has sufficient intensity to etch the surface thereby creating a stitch having reduced reflectivity.
It is possible, for a number of reasons, that one or more stitches are not properly etched. For example, the optics of the etching apparatus may become misaligned, dust particles may interfere with the incident beam of light, or the incident beam may not be properly focused upon the medium. It is critical for proper servo positioning that every stitch in each optical servo track be properly etched; one missing etch will ruin the entire disk. Thus, there exists a need for an apparatus and method for etching the surface of a magnetic medium and for verifying that proper etching occurred. The present invention satisfies this need.
Additionally, the width of the etched stitches is on the order of microns and must be maintained within tight tolerances. Stitch width can be affected by improper focussing of the incident beam. Thus, there also exists a need for an apparatus and method for imaging the etched spot on the surface of the medium and determining the size of the etched spot from the image in order to ascertain whether focus adjustment is necessary. The present invention satisfies this need as well.
It is also desirable to provide a method for aligning the incident beam to the center of rotation of the spindle in order to ensure accurately known radii of the servo tracks and to maintain accurate angular positioning of the stitches relative to a fixed angular index from the outer track to inner track. Such a method is further provided in accordance with the present invention.